1. Field of the Invention
This invention relates to an oven for the thermochemical treatment of metals such as, for example, steel or steel alloys. The thermochemical treatment is effected by ionic bombardment.
2. Description of the Prior Art
In general, the ovens usually employed for this purpose are designed to bring the gas used in the treatment process to a very low pressure, for example, a pressure of several mm Hg, and maintain the gas at that pressure. These ovens, in addition, include an anode and a cathode which acts as a support for the parts to be treated. The anode and cathode electrodes are connected across a high voltage direct current power supply circuit.
These ovens are usually operated in two different types of modes.
A first type or mode of operation consists in setting up a difference in potential between the anode and cathode such that, after a transition period, operation of the oven will be maintained on part of the voltage/intensity curve which may give rise to an electric discharge in the gas contained in the oven, close to the arcing regime. This part of the curve of operation is generally known as the "abnormal discharge zone". The curve mentioned heretofore is disclosed in the aforesaid copending application.
A second type or mode of operation consists in using high voltage current pulses instead of a direct current. The total energy of the high voltage current pulses has a predetermined value calculated so that it will not be possible to reach the zone corresponding to the arcing regime in the aforementioned voltage/intensity discharge curve.
It is clear that, in one instance as in the other, the processing temperature obtained is a function of the geometry of the parts. Consequently, one of the significant difficulties of ionic nitriding lies in the fact that only parts which have a very similar geometry can be readily nitrided simultaneously. However, with the prior art ovens, it has not been possible to treat articles with different geometric configurations in the same oven simultaneously.
Furthermore, a serious problem associated with thermochemical treatment processes which use ionic treatment is the problem relating to arc formation.
It is known, indeed, that mainly in the case of the first type of operation previously mentioned, the risk of accidental arc formation is relatively high because the operating point of the oven is close to the arc formation zone. In spite of the use of a protective system with this type of oven, and in which the protective system operates through arc detectors, the risk of deterioration of the parts is high.
Furthermore, this risk of accidental arc formations is multiplied if several parts having different geometries are treated simultaneously in the same oven. Indeed, it is known that arc formation is, among other things, a function of the geometry of the parts (tip effect, for example, etc. . . ).
It should also be noted that with the conventional ovens, there is a particular operating cycle which is carried out. The operating cycle in an ionic bombardment oven comprises the following sequences:
1. Charging of the oven under an atmosphere with oven open. PA1 2. Production of a vacuum inside the oven. PA1 3. Cleaning and passivation of the parts using a sequence of ionic bombardment with a passivation gas such as hydrogen. PA1 4. Cleanout operation by vacuum drawing. PA1 5. Introduction of a processing gas or gaseous mixture. PA1 6. Actual thermochemical treatment. PA1 7. Cooling under an atmosphere or immersion in an appropriate liquid. PA1 8. Removal of the parts from the oven.
It is clear that during a conventional thermochemical treatment, it is necessary to effect a continuous cleanout inside the oven. Taking into account the technology of the equipment, such continuous cleanout implies a substantial expenditure of energy.
In addition, because these ovens are opened and closed a large number of times, the surrounding air in the premises in which the ovens are located is polluted. Furthermore, the inside of the oven where all the successive operations are carried out may also be the site of gaseous mixing which may lead to explosions. The personnel assigned to these premises is therefore subjected to working conditions which are often difficult and relatively dangerous.
It is therefore the object of the invention to eliminate all these drawbacks.
Another object of the invention to provide for the simultaneous treatment of several parts having different geometrical configurations in one and the same processing oven.
A further object of the invention is to provide for the simultaneous treatment of several parts having different geometries without increasing the risk of arc formation.